1. Field of the Invention
The present invention generally relates to a digital watermarking technique. More particularly, the present invention relates to a digital watermarking technique for embedding or reading digital watermark data in digital data contents which represent an image or a sound. In addition, the present invention relates to a technique for statistical processing of read watermark data in a system using the digital watermarking technique.
It is easy to replicate or tamper fraudulently with multimedia production, and the easiness hinders an data content provider from sending data. In addition, some users may not use the data originated from the provider validly. Therefore, copyright protection is strongly needed for the multimedia production. The digital watermarking technique is effective in realizing the copyright protection. According to the digital watermarking technique, sub-data is embedded in data contents without being noticed by a user by utilizing redundancy of data such as of an image and a sound. The digital watermarking technique is used for protecting a multimedia copyright by embedding copyright information, a user ID and the like as the sub-data in secret, since it is difficult to separate the sub-data from the data contents.
2. Description of the Related Art
Conventionally, the following digital watermarking techniques are proposed.
According to a technique proposed in Japanese patent application No.9-57516, xe2x80x9cImage processing method and the apparatus,xe2x80x9d an image is subdivided into blocks larger than a 8xc3x978 block size which is used for common non reversible compression. Then, the size of the frequency coefficient which is obtained by discrete Fourier transform of the block is changed, the frequency coefficient being represented by a polar coordinate system and the size being a distance from the origin point of the polar coordinate system. As a result, sub-data can be read correctly even when the non-reversible compression is performed. In addition, the frequency coefficient is normalized within a range of predetermined values, is embedded, and read. In addition, weaker image processing is carried out on a complicated region as compared to a flat region. As a result, degradation of image quality which may be caused by embedding the sub-data can be suppressed and a tolerance to contrast changing is obtained. Further, as the value of the frequency coefficient to be changed becomes larger, the modification amount of the frequency coefficient becomes larger (the smaller the value is, the smaller the modification amount is) so as to suppress the deterioration of image quality more effectively. In addition, when subdividing an image into blocks, an image area which is smaller than one block is treated as one block by using an average pixel value and/or using a form symmetric with respect to a line repeatedly to compensate for the lacking image area. Moreover, the sub-data is constituted from the whole image after weighting data of each block. As a result, the sub-data is read correctly even when the image is partly edited and/or the image with many flat parts is non-reversibly compressed.
In addition, according to a technique proposed in Japanese patent application No.9-164466, xe2x80x9cInformation embedding method, data reading method and the apparatus,xe2x80x9d when embedding data into motion pictures, data embedding is carried out to components of a relatively low frequency region. Further, frequency conversion is carried out with a block size larger than a block size used for data compression, and, then data embedding is carried out. Moreover, an original image is used when data is read. As a result, tolerance to data compression is obtained.
Other conventional techniques are proposed in Japanese patent applications No.8-305370, No.8-338769, No.9-9812, No.9-14388, No.9-109924, No.9-197003, No.9-218467 and No.10-33239. The digital watermark method is also called data hiding, finger printing steganography, image/sound deep encryption and the like.
Elements for determining performance of the digital watermarking technique are as follows:
(1) quality of data contents in which the digital watermark is embedded;
(2) durability of the digital watermark which is embedded in the data contents when the data contents are manipulated;
(3) safety against intentional erasing of and tampering with digital watermark data, and
(4) reliability of the digital watermark data which is read from the data contents.
The digital watermarking technique is broadly divided into two methods. One method of gives meaning to a data value by quantizing. For example, by dividing a data value by a quantization value and dividing the result by 2, a bit data can be represented by the remainder. Another method embeds digital watermark data by using a spread spectrum method.
The above-mentioned examples are based on the former method. In terms of the method, there is a problem with respect to the above element (1) in that the digital watermark data embedded in the data contents may be perceived, or commercial value of the data contents may be lost by embedding the digital watermark data. With respect to the above element (2), the digital watermark data which is embedded in the data contents may be dissipated even when a general user uses the data contents in a normal way. Particularly, it is a difficult problem to achieve both elements (1) and (2) with enough performance in practical use.
In addition, there is a method of embedding the digital watermark data repeatedly in order to give durability to the digital watermark data against manipulation of the data contents. Specifically, according to the method, digital watermark data which is embedded repeatedly (which is called a watermark sequence hereinafter) is read from data contents, and, then, the digital watermark data is reconstituted by performing statistical processing. The watermark sequence has durability against deterioration and noise to some extent. However, if the data contents are encoded by high compression rates, it may become difficult to read the watermark sequence from the data contents. Therefore, it may become impossible to reconstitute the digital watermark data.
In addition, as for a digital watermarking system, accuracy for determining the presence or absence of embedded data is important. In addition, reliability of embedded data is important. The digital watermarking system generally has a mechanism for reconstituting correct digital watermark data even when sub-data embedded in the data contents is corrupted to a certain extent, since the digital watermarking system assumes various processing on the watermarked data contents. However, under present circumstances, it is impossible for the system to evaluate validity of reconstituted digital watermark data quantitatively. Therefore, the system does not have enough reliability.
It is a first object of the present invention to improve quality of watermarked digital data contents and to improve durability of digital watermark data against media processing of the watermarked digital data contents.
It is a second object of the present invention to evaluate quantitatively probabilities of cases that data contents which do not contain digital watermark data are wrongly judged as containing digital watermark data, and incorrect digital watermark data is read from watermarked digital data contents.
It is a third object of the present invention to separate a digital watermark data sequence, when reading watermarked digital data contents, from noise so that error bits which are included in the digital watermark data sequence can be reduced, thereby watermark data reading success rate being improved in comparison with the conventional method without changing an amount of digital watermark data and a method of embedding the digital watermark data.
The first object of the present invention is achieved by a method for embedding digital watermark data in digital data contents. The method includes the steps of:
receiving the digital data contents and the digital watermark data;
dividing the digital data contents into block data;
obtaining a frequency coefficient of the block data;
obtaining a complexity of the block data;
obtaining an amount of transformation of the frequency coefficient from the complexity and the digital watermark data by using a quantization width;
embedding the digital watermark data in said digital data contents by transforming the frequency coefficient by the amount; and
generating watermarked digital data contents.
The first object of the present invention is also achieved by a method including the steps of:
receiving the digital data contents and the digital watermark data;
dividing the digital data contents into block data;
obtaining a frequency coefficient of the block data;
obtaining an amount of transformation of the frequency coefficient from the digital watermark data by using a quantization width corresponding to the frequency coefficient, the quantization width being obtained beforehand according to a manipulation method of the digital data contents;
embedding the digital watermark data in said digital data contents by transforming the frequency coefficient by the amount; and
generating watermarked digital data contents.
According to the above-mentioned inventions, the amount of transformation of frequency coefficients is changed and/or the amount of transformation is increased or decreased according to the complexity of the digital data contents. Therefore, the quality of the watermarked digital data contents can be improved and the durability of digital watermark data against a manipulation of the watermarked digital data contents can be improved.
The second object of the present invention is achieved by a method for reading digital watermark data embedded in digital data contents, the method including the steps of:
receiving the digital data contents;
reading a bit sequence from the digital data contents;
calculating a probability of reading a bit xe2x80x981xe2x80x99 or a bit xe2x80x980xe2x80x99 in the bit sequence by using a test method on the basis of binary distribution;
determining the presence or absence of digital watermark data according to the probability; and
reconstituting and generating the digital watermark data from the bit sequence.
According to the above-mentioned invention, probabilities of the following cases can be evaluated quantitatively. The cases are that digital data contents which do not contain digital watermark data are wrongly judged as containing digital watermark data, and incorrect digital watermark data is read from watermarked digital data contents. In addition, the probability can be suppressed within a constant value.
The third object of the present invention is achieved by a method for reading digital watermark data from digital data contents in which each bit of digital watermark data is embedded a plurality of times, the method including the steps of:
receiving digital data contents;
reading a digital watermark sequence from the digital data contents;
performing soft decision in code theory by assigning weights to the digital watermark sequence with a weighting function;
reconstituting the digital watermark data from the digital watermark sequence.
According to the above-mentioned invention, the digital watermark data sequence is separated from the noise so that error bits which are included in the digital watermark data sequence can be reduced, thereby the digital watermark data reading success rate being improved in comparison with the conventional method. In addition, since weights are assigned to the digital watermark data sequence, the present invention is especially effective when the repeating number of watermark embedding is small.